Method of making a semiconductive ceramic body and a low voltage sparking device emboying same



R. L. COMER METHOD OF MAKING A SEMICONDUCTIVE CERAMIC BODY AND A LOW VOLTAGE SPARKING DEVICE EMBODYING SAME 1 Filed May 16, 1960 Oct. 29, 1963 INVENTOR. Wit/20rd C Comer ATTORNEY 3,108,905 WTHOD OF MAKING A SEMICONDUCTIVE CERAMIC BODY AND A LOW VOLTAGE SPARK- ING DEVICE EMBODYING SAME Richard L. Comer, Grand Blanc, Mich, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed May 16, 1960, Ser. No. 29,472 12 Claims. (Cl. 117-2l3) combustion engines of the automotive and aviation type and those normally called on for intermittent use such as the igniter plugs used in jet engines, oil burners and diesel type engines generally employ a. pair of electrodes separated by an air gap. In an attempt to overcome the problems inherent in such sparking devices, i.e., fouling and electrode erosion, much development work has been done to develop a low voltage creep gap type device. In such device the electrodes defining the spark gap are positioned in such manner with reference to the insulator body separating the electrodes as to place the insulator surface adjacent the gap in abutment with the electrodes. The insulator surface adjacent the electrodes is formed in such manner as to possess electrically conductive properties to enable the spark to move across the surface in stead of through the air gap. While many types of semiconducting bodies and surfaces have been developed for use with low voltage sparking systems, most semiconductors have been found to erode very rapidly due to channeling and excessive porosity.

Accordingly, it is an object of my invention to provide a refractory semiconducting insulator body enabling low sparking voltage and having good resistance to spark erosion. It is a further object of my invention to provide a low voltage sparking device having a refractory semiconducting insulator surface abutting the spark gap and having properties of good thermal stability and resistance to spark erosion. It is a further object of my invention to provide a method for forming an insulator body having a surface thereof provided with a semiconductive material to form a stable erosion resistant low voltage semiconductive coating. It is a further object of my invention to provide a method for forming a high alumina insulator body having a surface thereof provided with a semiconductive material to form a stable erosion resistant low voltage semiconductive coating.

The objects of my invention may be achieved by applying a coating of firom 10-40% copper and 6090% chromic oxide on the insulator surface followed by firing the coated insulator at a temperature of from about 2400 F. to about 2500 F. and soaking the insulator at the elevated temperature for a period of at least 15 minutes.

Other objects and advantages of my invention will be- I come apparent from consideration of the following disclosure, taken in connection with the accompanying draw- 3,108,95 Patented Oct. 29., 1963 ice 2 ing showing a sparking device in partial cross-section and embodying the principles of my invention.

Referring to the drawing, there is shown an igniter plug 1 comprising a shell '3 within which there is positioned a center electrode subassembly 5, the subassembly being formed and cooperating with the shell in a manner well known in the prior art and forming no part of my invention. As shown, the center electrode subassembly 5 comprises a nonconducting refractory oxide insulator body 7 having positioned therein a center electrode 9 at the lower end thereof and a terminal screw member 11 at the upper end thereof. The center electrode 9 is provided with a button-like sparking end 13 which is positioned within and annulamly spaced apart from the inwardly turned lower end 15 of the shell *3, which end 15 constitutes the ground electrode. Positioned between the lower end of the insulator 7 and the gap defining electrodes 1'3 and 15 are annular gaskets 17 and 19 which are dimensioned so as to be co-extensive in diameter with the electrodes upon which each is positioned to define a spark gap of uni-form width. It should be understood that the sparking device described has been shown merely by way of example, my invention being equally applicable to devices of difierrent design.

As is clearly shown on the drawing, the lower end surface of the insulator abutting the spark gap is provided with a coating Z1 having semiconductive properties to enable sparking at voltages of 1500 volts and lower and being resistant to spark erosion at temperatures as high as 2000 F. during normal operating conditions. I have found that coatings having the desired physical and electrical properties may be formed by using a composition containing about 10-40% by weight copper (Cu) and about -90% by weight chromic oxide (-Cr O The preferred range of composition is that containing about 30- 40% by weight of Cu, the best thermal and electrical stability as well as the best resistance to erosion being obtained by using a composition containing about 30% Cu and about Cr O In conducting my investigations I have found that a two coat application of the identified composition produced results which were better than those obtained with one coat in that the sparking voltage was found to be lower. 1 have also discovered that the final coating obtained by use of two or more separate coating applications on an insulator body surface which had been treated by any suitable etchant such as hydrofluoric acid produced the best bond to ceramic and the best resistance to spark erosion.

In the following Tables I to V, inclusive, there are shown the test data using the coatings of my invention and demonstrating the results obtained by using multiple coats as well as varying compositions, the compositions being applied to both prepared and unprepared insulator surfaces and being fired under different conditions. The coatings shown in Table I were applied to insulator surfaces using a firing schedule of about 12 hours to raise the temperature to about 2400 F., the specimen being soaked at the elevated temperature for about 3 hours. The data shown in Table II results from using coatings fired over about a 5 hour period to raise the temperature to about 2500 F. with about a 15 minute soaking period at the elevated temperature. The data shown in Tables i.- III, IV, and V result from using the same firing schedule as with reference to the data shown in Table II except that the soaking period was 30 minutes. Each of the multiple coats referred to in Tables IV and V were separately fired using the firing schedules noted above. It may be observed that satisfactory results are obtained by using temperature and the time conditions which are inversely related to each other.

RESULTS OF TESTS ON Cu-Cr O COATINGS AP- Table I Composition Sparking voltage Num Insulator ber Surface Initial After 5 min. at coats preparation 1,500 v., 4. mid. Cu CrrOa .1 mid. 4 mid. .1 mid. 4 mid.

10 90 1 None 1, 800 1, 800 1, 600 1, 400 20 80 1 800 700 1, 200 700 30 70 1 1, 300 800 1, 500 1, 000 40 60 1 900 500 1, 000 500 50 50 1 1, 100 500 1, 100 1, 200 60 40 1 1. 100 400 900 400 70 30 1 900 500 300 200 80 20 1 1, 200 500 400 200 90 10 1 1, 100 500 400 400 Table II Composition Sparking voltage Num- Insulator ber surface Initial After 5 min. at coats preparation 1,500 v., 4 mfm, C11 C1303 .1 mid. 4 mid. .1 mid. 4 mid.

80 70 1 1, 400 1, 100 1, 300 700 40 60 I 1, 200 1, 100 800 700 50 5O 1 1, 000 800 900 700 60 40 1 800 800 800 700 70 30 1 1, 200 900 1, 300 900 S 20 1 1, 400 1, 200 1, 500 l, 200

Table III Composition Sparking voltage N um- Insulator ber surface Initial After min. at coats preparation 1,500 v., 4 mig. Cu CrzOs .1 mid. 4 mid. .1 mid. 4 mid.

3O 70 1 None 1,400 1, 100 1,300 1, 200 40 60 1 d0. 1, 200 900 1, 100 900 50 50 1 d0- I, 100 800 1, 100 700 60 40 1 d0- 1. 300 700 1. 300 800 70 30 1 do 1.400 900 1,100 1,100 80 20 1 do 1, 400 900 1, 200 1.100

Table l V Sparking voltage Composition Insulator Number surface Initial After 3 min. at 1,500 Erosion Coats preparation v., 4 mid.

Cu CH0; .1 mid. 4 mid. .1 mid. 4 mid.

90 2 550 950 550 Light. 2 .500 950 500 Acceptable. 70 2 450 550 450 Do. 40 60 2 450 550 450 D0. 2 400 1, 200 400 Severe. 40 2 450 850 450 D0. 30 2 450 1, 100 450 D0. 20 2 500 900 500 D0. 10 2 400 600 400 Do.

Table V Sparkin voltage Com osition Insulator p Number surface Initial After 3 min. at 1,500 Erosion Coats preparation v., 4 m

Cu OrzOa .1 mid. 4 mid. .1 mid. 4 mid.

10 90 3 950 700 950 700 Very light. 20 80 3 1,050 600 1. 050 600 Do. 30 70 3 650 450 650 450 Do. 40 60 3 Etched in 800 450 800 450 Do. 50 50 3 HF, 10 950 400 950 400 Severe. 60 40 3 min. 850 400 850 400 Do. 70 30 3 700 500 700 500 Do. 80 20 3 950 450 950 450 D0. 3 750 300 750 300 Do.

I have found that the coatings having the desired physical and electrical properties as stated above are obtained by using the following processing schedule. The desired amounts of Cu and Cr O are weighed out and then thoroughly mixed. For ease of application the raw materials are mixed with water or other suitable volatile solvent such as alcohol, methyl ethyl ketone, turpentine, etc., to form a slurry or slip of such consistency as to be readily applied to the insulator surface. The coating may then be applied to the desired surface by brushing methods or by any other suitable technique, such as dipping or spraying. I have found that satisfactory applications of coatings can be made by dipping a small brush in the coating slip and placing the brush in contact with the rotating insulator surface so that the slip may be pulled from the brush. After air drying at room temperature, the coatings are fired in a non-reducing or air atmosphere.

In order to achieve a satisfactory bond between the coating and the insulator as well as a hard coating, I have found that the firing schedule should be such as to raise the temperature to from about 2400 F. to about 2500 F. during a period of from about 5 hours to 12 hours with a soaking period at the elevated temperature of from about 15 minutes to about 3 hours. In this regard I have found that coatings fired to temperatures of 2100" F., 2200" F., and 2300 F. were immature and soft and unsuitable for the purposes intended. Similarly, I have found that etching of the insulator surfaces to be coated prior to coating resulted in a superior bond of the coating to the insulator. Such etching may be suitably achieved by using hydrofluoric acid and subjecting the insulator surface to the etchant for a period of from about 5 to 10 minutes.

As pointed out above and as shown by the results tabulated, I prefer to use a coating composition of about 30% copper and 70% chromic oxide applied to the surface of the insulator which has been prepared by etching in the manner indicated. The preferred firing schedule raises the temperature to about 2500 F. over a period of about 5 hours, the elevated temperature being held for a period of about 30 minutes. While a single coating with even an unetched surface produces the desired results insofar as physical and electrical characteristics are concerned, I prefer to use multiple coatings applied to an etched insulator surface, each successive coating being separately dried and fired in the manner indicated.

While I do not desire to be bound by any statements of theory concerning the mechanics or chemistry involved in my invention, it appears that the fired coatings in the useable ranges disclosed herein, about 10-40% Cu and about 60-90% Cr O contain Cr O cubic chromic oxide, Cu Cr O and CuCr O the amounts of each varying dependent upon the composition of the coating slurry. Similarly, from the physical standpoint, the reference to coatings and other similar descriptive terms as used herein is not to be construed as limited to merely a layer on the surface of the insulator but to also include the diffusion of the coating composition into the surface of the insulater. the cn-cr o systems, it should be noted that numerous other systems have been investigated and have been proven to be unsatisfactory. In this regard, it is significant to note that the systems most closely related to that of my invention, the systems CuO-Cr O and Cu O-Cr O were entirely unsuitable for the purposes intended in view of the extremely severe spark erosion characteristics of their coatings. It should also be noted that the Cu-Cr O system described herein was found to wet the surface of a high alumina insulator Whereas chromic oxide compositions using niobium or chromium metal did not.

I have also found that variations in chemical and electrical properties in the Cu-Cr O system have been due to oxidation of the copper powder on standing in air atmosphere. To minimize such oxidation, the copper powder is heated, as needed, in a reducing atmosphere, i.e., hydro gen, at about 1300 F. for a period of about 15-30 minutes. The material is then sized to pass through a 200 mesh screen and used as described above.

In forming the sparking devices of my invention, the insulator having a semiconductive surface formed in the manner described above is assembled with the center electrode to form the insulator subassembly. The center electrode is in good electrically conductive contact with the semiconductive surface, such contact being either direct or through a metal gasket as shown in the drawing. The insulator subassembly is then assembled within the spark plug shell in a conventional manner, the ground electrode portion of the shell being in similar good electrically conductive contact with the semiconductive surface of the insulator, in the manner described.

It is apparent from the foregoing description that I have provided a method for forming a semiconductive coating on an insulator surface for use broadly as well as more particularly for use in low voltage creep gap sparking devices. Also, as shown, the methods of my invention enable the fabrication of a low voltage sparking device capable of operating at voltages of from about 400 volts toabout 1500 volts while at the same time being electrically stable and resistant to spark erosion and thermal stresses. While other embodiments of my invention may be apparent to those skilled in the art, it is intended that such embodiments fall within the scope of my invention and as described above and as set forth in the attached claims.

What is claimed is:

l. The process of making a ceramic insulator having a semiconductive surface, which comprises the steps of applying to the surface of the insulator a composition consisting essentially of about 1040% copper and about 60-90% chromic oxide, firing the treated insulator in a non-reducing atmosphere to a temperature of from about 2400 to 2500 F., and maintaining the insulator at the elevated temperature for a period of at least about 15 minutes.

Further, while I have limited my description to 2. The process as set forth in claim 1 wherein said composition consists essentially of about 3040% copper and 60-70% chromic oxide and the firing temperature being about 2500 F.

3. The process as set forth in claim 2 wherein the composition consists essentially of about 30% copper and 70% chromic oxide, the treated insulator being fired in an oxidizing atmosphere.

4. The process of making a ceramic insulator having a semi-conductive surface adapted for use in a low voltage creep gap sparking device, which comprises the steps of applying to the predetermined surface of the insulator a composition consisting essentially of about 10-40% copper and about 6090% chromic oxide, firing the treated insulator in a non-reducing atmosphere to a temperature of from about 2400 to 2500 F. over a period of from about hours to 12 hours, and maintaining the insulator at the elevated temperature for a period of from about 15 minutes to about 3 hours, the resulting surface enabling sparking at from about 400-1500 volts while being highly resistant to spark erosion.

5. The process as set forth in claim 4 wherein said composition consists essentially of about 50-40% copper and 60-70% chromie oxide.

' 6. The process as set forth in claim 5 wherein the insulator surface is treated by etching prior to application of the composition.

7. The process as set forth in claim 6 wherein said composition is applied in at least two separate coatings, each coating being fired separately under substantially the same conditions.

8. The process as set forth in claim 7 wherein the composition consists essentially of about 30% copper and about chromic oxide.

9. The process as set forth in claim 8 wherein the coated insulator is fired to a temperature of about 2500 F. over a period of about 5 hours, the insulator being maintained at the elevated temperature for a period of at least about 15 minutes, the copper being in the form of less than 200 mesh powder which is prepared for use by heating in a reducing atmosphere for a period of at east 15 minutes and at a temperature of about 1300- F.

10. A nonconducting refractory oxide insulator adapted to have a surface portion thereof in abutment with sparking electrodes adjacent the spark gap to bridge the gap between the electrodes, said insulator surface being formed by the process of claim 4.

11. A non-conducting refractory oxide insulator adapted to have a surface portion thereof in abutment With sparking electrodes adjacent the spark gap to bridge the gap between the electrodes, said insulator surface being formed by the process of claim 6.

12. A nonconducti-ng refractory oxide insulator adapted to have a surface portion thereof in abutment with sparking electrodes adjacent the spark gap to bridge the gap between the electrodes, said insulator surface being formed by the process of claim 9.

References Cited in the file of this patent UNITED STATES PATENTS 2,578,754 Smits Dec. 18, 1951 2,684,665 Fognola July 27, 1954 2,861,014 Sheheen et al. Nov. 18, 1958 2,953,704 Harris Sept. 20, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,108,905 October 29 1963 Patent No Richard I... Comer s in the ab ters Paten ove numbered pat- It is hereby certified that error appear ction and that the said Let I: should read as ent requiring corre the right-hand corrected below.

Columns 3 and 4, in Tables 11 and Ill, sub-titles under "Sparking voltage should appear as shown below instead of as in the patent: After 3min, at l 500 v. 4 mid Signed and sealed this 28th day of April 1964.

(SEAL) Attest: ERNEST W SWIDER EDWARD J, BRENNER Commissioner of Patents Attesting Officer 

1. THE PROCESS OF MAKING A CERAMIC INSULATOR HAVING A SEMICONDUCTIVE SURFACE, WHICH COMPRISES THE STEPS OF APPLYING TO THE SURFACE OF THE INSULATOR A COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 10-40% COPPER AND ABOUT 60-90% CHROMIC OXIDE, FIRING THE TREATED INSULATOR IN A NON-REDUCING ATMOSPHERE TO A TEMPERATURE OF FROM ABOUT 2400 TO 2500*F., AND MAINTAINING THE INSULATOR AT THE ELEVATED TEMPERATURE FOR A PERIOD OF AT LEAST ABOUT 15 MINUTES. 